1. Field of the Inventions
The present invention relates to radio frequency (RF) tuners and specifically to integrated RF tracking filters.
2. Background Information
Following the integration trend in silicon, broadband tuners have also been integrated onto a single silicon chip. Many new applications such as Digital Video Broadcast-Handheld (DVB-H), Universal Serial Bus (USB) TV tuners for the personal computer, and dual tuners for Digital Video Recorder (DVR) have placed a premium on reduction of power. For example, low power DVB-H tuners can enable TV tuners to be integrated into wireless applications. However, for a wireless application low power consumption is extremely desirable as it leads to longer battery life. Low power consumption is very desirable for applications such as USB TV tuners to fit within the power supply capabilities of the USB interface. Furthermore, lower power consumption means lower heat dissipation requirements which enable applications that have packaging heat dissipation constraints such as dual tuners in DVR applications for cable, satellite, and terrestrial TV.
The power consumption in silicon RF tuners can be attributed to many sources. First, it requires tuners to exhibit a high linearity to be able to deal with various standards. In particular, the third order intercept point (IP3) must be within specified values. Depending on the application, various standards govern the lower bound of the IP3 such as Advanced Television Systems Committee (ATSC), National Television System Committee (NTSC), Data-Over-Cable Service Interface Specification (DOCSIS), and Next Generation Digital Video Broadcasting over Satellite (DVB-S2). High linearity requirements are necessary as they allow the tuner to maintain performance in many different environments with strong adjacent channel interferers, which in some cases are many times stronger than the desired signal. Lots of current and many popular feedback techniques are typically required to keep the IP3 high enough to meet the requirements of these TV standards.
Another dominant source of power consumption in silicon RF tuners is the receiver architecture. Traditionally super-heterodyne receivers, have been used for terrestrial and cable television applications due to their immunity to interference from local oscillator (LO) harmonics, by converting the incoming RF signal into an intermediate frequency (IF) signal and then extracting the baseband (or information) signal from the downconverted IF signal. However, super-heterodyne receivers consume a great deal of power due to its two frequency conversion stage architecture.
A direct conversion receiver which converts an incoming RF signal directly to a baseband signal consumes less power than the super-heterodyne receiver due to the savings in having a single stage and due to the need for only one downconversion step. FIG. 1 shows a basic direct conversion receiver. For clarity, many typical components are excluded from the diagram. An RF signal is received through antenna 102. Often the signal is boosted by low-noise amplifier (LNA) 104. The signal is then downconverted using mixer 106 with a signal produced by LO 108. The result is baseband signal 110 which is then further processed.
However, due to the proximity in frequency of the local oscillator signal to the input signal frequency a direct conversion receiver in broad band tuner applications is susceptible to undesired adjacent and alternate channels at the frequency of LO harmonics. For example in an analog terrestrial television application, the broadband tuner accepts frequencies ranging from 50 MHz to 850 MHz. To illustrate this point, suppose the tuner is set to receive at the low end of this range at 50 MHz. The LO would also be set to 50 MHz, but harmonics such as the 3rd and 5th harmonics could also be introduced into the mixer at 150 MHz and 250 MHz. (Typically, differential signals are used so even harmonics cancel out leaving only the odd harmonics as a concern.) This would result an undesired effect that any signals at 150 MHz and 250 MHz are downconverted to a baseband signal as well as the desired signal at 50 MHz. While the local oscillator (LO) harmonics generally are weaker than the LO fundamental frequency, it is not uncommon that the unwanted signals at the 3rd or 5th LO harmonic frequencies are stronger than the desired RF signal. The net result is that the interference from the LO harmonics can be quite significant and can desensitize the receiver.
One approach to remedy this situation is to apply a bandpass filter around the desired frequency to the received signal. In order to maintain the desired sensitivity and selectivity for the receiver, the bandpass filter should have a high Q in order to attenuate the adjacent channels and to maintain a large dynamic range. The RF tracking filter should have a programmable center frequency to follow the desired channel frequency.
FIG. 2 shows a direct conversion receiver with an RF tracking filter. Receiver system 200 is identical to receiver system 100 except between LNA 104 and mixer 106 is RF tracking filter 202. Ideally, the RF tracking filter would permit only signals in desired channel to pass. While the LO harmonics still exist and reside at the same levels as in system 100. RF tracking filter 202 attenuates the input RF signal at the corresponding LO harmonic frequencies thereby reducing or eliminating undesired baseband signals downconverted from the LO harmonics.
One of the major reasons for the high linearity requirements is because non-linearity in a receiver leads to inadvertent frequency mixing, that is signals residing in different channels can get mixed together some of which can result in interference in the desired channel. As a result, since an RF tracking filter attenuates undesired channels over the enter frequency band. Thus, by implementing an RF tracking filter, the linearity requirements of the subsequent receiver stages can be relaxed. In particular, the amplifier stages can be implemented with much lower IP3 specifications reducing the total power dissipation of the receiver. More specifically, a silicon terrestrial tuner with an RF tracking filter can achieve much improved composite tripe beat (CTB) and composite second-order beat (CSO) performances and henceforth can obtain a large signal to noise plus distortion ratio (SNDR) at its output.
The benefits of the RF tracking filter is tempered by the challenges introduced by implementing the RF tracking filter in silicon. Chief among the challenges are minimizing the chip area and the power consumption, in particular the amount of current required. To effectively reduce the current consumption of TV receivers, the RF tracking filter should be fully integrated into the RF front-end (RF front-end in general stands for a receiver front-end section which includes all blocks before the first frequency conversion) of a tuner because: discrete RF tracking filters exhibit insertion loss so it requires more gain of the low noise amplifier and this will result in more current consumption of the low noise amplifier; Further it also requires both an input buffer and an output buffer to interface with the discrete RF tracking filter but both buffers consume currents for linearity. Ideally, the RF tracking filter should have a very wide dynamic range with a low noise figure, high IP3 and a minimum amount of voltage gain required to maintain the overall sensitivity limited by the noise figure of the subsequent amplification stages. Specifically, a low noise figure is desired because, a tracking filter is located at the RF front-end before the RF mixer. Furthermore, a high IP3 is needed because the received RF signals are amplified by a low noise amplifier located before a RF tracking filter. A gain produced by the tracking filter will make the noises of the following circuit blocks contribute less to the overall tuner noise figure because their contribution to the overall noise figure are divided by the accumulated gain including the gain from the tracking filter. For wide band applications, the tracking filter should also be tunable over a very wide band, for example from 50 MHz to 850 MHz as given in an example above. The gain should remain flat over the entire tuning range with minimal band-pass ripple.
There have been many different topologies implemented in the past as RF Tracking Filters for various receiver designs. While suitable for some applications, these different topologies have significant drawbacks making them unsuitable for or at a minimum difficult to use in wide band receiver applications.
One topology used in the past is a discrete, passive LC ladder tracking filter as taught in U.S. Pat. No. 6,453,157. The architecture is a basic ladder circuit comprising inductors and capacitors (hence LC). While this topology provides attenuation up to 50 dB, it suffers from a discrete implementation comprising components to bulky to be efficiently integrated into a silicon tuner. In the embodiments taught by U.S. Pat. No. 6,453,157, six 33 nH inductors are used which would occupy an extremely large amount of area on a silicon chip. Additionally, there is a loss of approximately 5 dB by using a purely passive component. A loss introduced by the tracking filter results in a higher noise contribution from the following blocks of a receiver and therefore makes the overall noise figure of the tuner higher. Finally, the topology has a relatively narrow tuning rage of 400-470 MHz falling well short of the require range for terrestrial and cable TV applications.
Another tracking filter topology is the transformer-based, Q-enhanced band-pass filter. A Q-enhanced band-pass filter is built from a series-C coupled sequence of resonators. FIG. 3A shows a basic series-C coupled resonator band-pass filter. A plurality of resonators 302 are coupled through a plurality of capacitors 304 connected in series. In the most basic form the resonators are simple LC resonators as shown in FIG. 3B comprising inductor 310 and capacitor 312.
The Q-enhanced band-pass filter improves upon the basic series-C coupled resonator band-pass filter, by using the resonator shown in FIG. 3C. The addition of transistors 320 and 322 enables the filter to introduce gain to compensate for any loss in the circuit and the ability to tune the frequency. The gain allows for a flatter band pass response.
While this design resolves some of the deficiencies of previous designs, it still suffers from several weaknesses that make it impractical for integrate silicon broad band applications. As an example, Gee, et al. (“CMOS Integrated LC RF Bandpass Filter with Transformer-Coupled Q-Enhancement and Optimized Linearity” Gee, W. A; Allen, P. E. Proceedings of IEEE International Symposium on Circuits and Systems 2007, P. 1445-1448) shows that the dynamic range is limited which results in a high noise figure and a low IP3, insufficient for wide band receiver applications. In addition this design does not provide adequate tuning range for the wide band receiver applications. Further, the presence of inductors in the form of transformers is too costly in terms of chip area.
Another design for a tracking filter is the integrated tunable Gm-C bi-quad filter as disclosed in U.S. Pat. No. 6,915,121 which overcomes the drawbacks of the previous filter designs by eliminating the need of on chip inductors. However, the disclosure does not discuss any specific dynamic range performance or tuning bandwidth coverage for comparison with other topologies.
A fourth tracking filter topology employs the use of Micro Electro-Mechanical Systems (MEMS) structures. An embodiment described by K. Entesari, et al. (“A 12-18 GHz Three-Pole RF MEMS Tunable Filter” K. Entesari, G. Rebeiz, IEEE Transactions on Microwave Theory and Techniques, Vol. 53. No. 8, August 2005) claims a very wide 40% tuning bandwidth from 12 to 18 GHz with good attenuation as high as 50 dB for adjacent channels and very high input referred IP3 (IIP3)≧+37 dBm. However, there are several drawbacks that prohibit use of the RF MEMs filters for cable, terrestrial, and satellite TV bands. The first limitation is the center frequencies of the bands, which are much higher than the desired 50 to 850 MHz for TV applications. Secondly, the filter requires an extremely large chip area of 32 mm2, which is nearly twice the size of most RF Silicon tuner chips used for these applications and would nearly triple the wafer costs. Also, the filter requires very expensive wafer post processing steps beyond conventional SiGe BiCMOS or CMOS technologies to add the MEMS filter structures. Lastly, the filter is passive and therefore not capable of providing any gain to limit the noise figure contribution of the back end receiver, making it impractical to be integrated into the RF front end of the tuner. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies.